While multistrike ribbons were widely used in the past and were composed of a fabric impregnated with a color transfer material and designed to run back and forth between the printing element and the platen upon which a sheet of paper or the like was supported, with the advent of word processing equipment, electrical typewriters, multicharacter printing elements, type wheels, type balls and the like, and with the desire for a high degree of reproducibility in the transferred impression, so-called disposable ribbons or film ribbons were developed.
Such ribbons, which originally could be used only once and were then discarded, were generally composed of a synthetic resin film or foil which was struck on the back by the print element, e.g. a type face or some other mechanical impact device, and had a color transfer layer on its opposite face whereby color was transferred to the receiving sheet.
In spite of the fact that such disposable ribbons were comparatively inexpensive, it nevertheless was desirable to reduce the cost still further by providing these ribbons with overstrike or multistrike capability, i.e. by permitting a region which had previously transferred color to the paper, to be reused and again transfer color so that each region of the ribbon could sustain a multiplicity of impacts. Generally speaking, in overstrike or multistrike ribbons, each region which is struck overlaps a previously struck region and is, in turn, overlapped by a subsequently struck region so that eventually each portion of the ribbon along with the characters strike the same, receives a multiplicity of impacts or strikes. Such ribbons are therefore also known as overlapping strike ribbons.
Overstrike, overlapping strike and multiuse disposable ribbons generally are advanced through an increment which is a predetermined fraction of a normal type width per impact so that it is common, for example, to incrementally advance the ribbon in steps of one third or one fifth of the normal type width so that each impact predominantly overlaps previous impact regions and only to a small extent involves a fresh region of the ribbon. By comparison with single-strike ribbons, therefore, for given lengths the ribbon must be replaced only one third to one fifth as often, and can sustain 3.3 or 5 impacts at each element of the impact area of the ribbon.
Naturally, the color transfer capability of the ribbon must be such that not all the color is transferred with the initial impact and sufficient color is retained for the subsequent impacts and further that the subsequent impacts can transfer enough material of sufficiently deep intensity and resolution.
The term "resolution" will be used herein in the same manner as "edge definition" or "edge sharpness", all of these terms meaning the ability to distinguish the transferred character from the background by an extremely sharp edge at the boundary of the character.
Overstrike ribbons also allow the complex mechanisms for ribbon reversal to be eliminated.
The aforedescribed prior applications, the art of record therein, U.S. Pat. No. 3,336,150 and U.S. Pat. No. 3,682,683 and Printed German Application No. 2,418,066 describe color transfer materials in which a transfer layer carrying the pigment is provided on a relatively thin support layer. This literature constitutes the best art currently known to us and having a bearing on this application.
Normally, overstrike ribbons consist of a thin carrier foil, to which the color-releasing layer is applied in the form of a matrix of a synthetic resin binder and a color paste dispersed therein. The color paste generally includes an oleaginous substance which does not interact with the support and may be referred to hereinafter as a pigment or color-displacing oil and the pigment which can be soluble or otherwise homogeneously distributed in the oil.
To produce this color transfer coating, a solution of the binder in a solvent, which is also a solvent for the oil, is applied to the carrier foil and the solvent phase eliminated by drying, i.e. the solvent is evaporated. This distributes the oil in numerous microdroplets which ideally intercommunicate with one another, within the hardened matrix formed by the resin or binder.
The color transfer coating is thus effectively a foam or sponge layer from which, at each impact or strike, a portion of the coloring material and oil is caused to extrude from the pores onto the paper and, especially where the pores intercommunicate, the thus discharged pores can refill from adjoining regions with the oil-pigment mixture.
An important desideratum for such overstrike ribbons is that with each impact, the quantity of pigment per unit area which is expressed from the color transfer layer should be the same for all regions and all subsequent impacts within the number of overstrikes for which the ribbon is designed.
This guarantees that the color intensity will not diminish from impact to impact, i.e. between the first strike and the last overstrike, that the image transfer at each stroke will be uniform over the entire character and free from more dense and less dense regions, and that the entire typescript will be uniform.
The oleaginous material which was used in the past was generally castor oil, neat's-foot oil, peanut oil, glycerine triolein or corresponding natural or synthetic oils. These substances were found to suffice with overstrike ribbons allowing five overlapping impacts.
To reduce the ribbon cost still further, yet greater overstrike capabilities are desirable and hence it is desirable to step the ribbon by substantially less than one fifth of the width of the type face. Conventional ribbons could not be utilized effectively for larger numbers of strikes without developing a mottled imprint most noticeable when the print involves underscoring.
In practice, therefore, when attempts were made with the prior art systems to utilize six or more strikes per region of the overstrike band, significant color fading was observed with the sixth strike and it was difficult to ensure for the sixth strike a sufficient supply of color.
It was proposed, in this connection, to increase the thickness of the band by increasing the thickness of the color transfer, thereby intending to provide additional coloring matter. This approach had little value because the increased thickness of the color transfer layer resulted in a decrease in the length of the ribbon for a standard spool and an ultimate reduction in the number of impressions which could be obtained from a given ribbon.
Even more significantly, when the color transfer layer becomes unduly thick, the color transfer therefrom from one strike to the next does not remain constant and hence the typescript is a mottled product.
Attempts have also been made to increase the proportions of the coloring paste in the color transfer coating. This has not been found to be practical since the formation of an effective sponge layer requires a certain minimum ratio of color paste to binder which must be maintained.
Finally, it may be noted that even efforts to increase the coloring power of the color paste did not work effectively. When, for instance, the coloring power was increased by raising the pigment concentration, the paste lost its flowability.
Efforts were also made to provide the color in the oil phase utilizing hydrophobic oil-soluble dyestuffs. This did not lead to effective results since even the hydrophobic or oleophilic dyestuffs were soluble in the earlier oleagenic substances only to limited degrees without sufficient increase in the coloring depth.